child_process: improve input validation

This commit applies stricter input validation in
normalizeSpawnArguments(), which is run by all of the
child_process methods. Additional checks are added for spawnSync()
specific inputs.

Fixes: https://github.com/nodejs/node/issues/8096
Fixes: https://github.com/nodejs/node/issues/8539
Refs: https://github.com/nodejs/node/issues/9722
PR-URL: https://github.com/nodejs/node/pull/8312


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>